Can we ignore for a few years for it to be resurrected with "I have the same problem" or "what was the solution"

While a joint is ok, I'm not sure a reputable garage would use one. By the time the joint was made a complete pipe could be installed. A pain on axle stands to get in but probably easier on a ramp.

With everyone ignoring it this is the thread to finally come out of the closet to.

Depending on level of security a money tin might be ok. Leaving out of sight isn't always best as anyone who has access to the car,eg mechanics, may find it and you possibly wouldnt know straight away.

Something like a gun box would work. Possibly an ammo box would be smaller

There is the option for a second lighter for the rear passenger, the cable could be there under the cubby box. You can then easily fish cable where you want it, even inside the cubby box.

Any newer diesel the dpf would be a consideration, do you drive enough to keep it clean and I would factor in replacement cost.

Could be engine mounts.

Do the sensors plug in independently to the module? If so does the issue follow the sensor or where they are plugged into. Try just one sensor at a time. Maybe a damaged cable that's picking up interference, you could try oscilloscope on the sensors

Top up using syringe, chemist can supply them for kids medicine. Try bleading it

So you can't turn the key, that is mechanical on the steering column. You may need to remove the ignition to free it.

Did it knock before the clutch change?

Yes, could be useful although the head unit is close, unless you have the need to control it from the back seat.

I considered adding push buttons may be by the window switches on the centre console for other functions, but volume is all I use really

My opinion is the early x5 looks dated sometimes which may affect resale value.